sd-bus: add comment and test in sd_bus_path_decode() for empty string

sd-bus: add comment and test in sd_bus_path_decode() for empty string

3970	        e = object_path_startswith(path, prefix);
(gdb) p path
$1 = 0x55c5a166f768 "/org/freedesktop/portable1/image"
(gdb) p prefix
$2 = 0x55c59ffc2928 "/org/freedesktop/portable1/image"
(gdb) p e
$1 = 0x5581a1675788 ""

This can be a bit confusing in certain cases, so add a comment and a
test to make the behaviour clearer and explicit.